Have you noticed how many "calendars" on the 'Net are really just passive representations? An awful lot of them are just HTML or PDF versions of what you used to get in paper newsletter- a static "printout" of dates and times, that isn't of any use to anything else on your computer.
On the other hand, some organizations do make an effort, providing each event with a link that causes the event to open in your local calendaring program, usually by providing a link to an iCalendar file.
The difference in usability between the two is really painfully obvious: picture yourself going to the school website of your kid, and on that website you find a link labelled "school calendar". Great, you think, I'll add all of Little Johnny's football practices to the family calendar. You click the link, only to find out that it's a PDF, and if you want to add those items, you're going to have to retype each and every one of them. Not very customer-friendly, I'd say. If they had opted to provide the "active" link to an .ics file, you could select ech of the events and add them to your calendar where they'd be most useful.
Vendors are starting to get serious about 'shared calendars" and this is a small but important step - they need to provide ways for all of the organizations in our lives to easily give us these active calendar links.